1. Field of the Invention
The present invention relates to an improved image sensing device. More particularly, the present invention relates to a solid-state image sensing device. In more detail, the present invention relates to an improved solid-state image sensing device having a photoelectric converting element for outputting an electric signal according to an amount of incident light, and an image sensing device including the solid-state image sensing device.
2. Description of the Related Art
A solid-state image sensing device is small, light weight, and of low-power consumption. Moreover, image distortion and burning may not occur in the device, and the device is capable of dealing with environmental conditions such as vibration and magnetic field. Further, since the device can be manufactured by the step which is common with or similar to that of LSI (Large Scale Integrated Circuit), its reliability is high and it is suitable to mass production. For these reasons, solid-state image sensing devices where pixels are arranged linearly is used widely for facsimile machines and flat bed scanners, and solid-state image sensing devices where pixels are arranged in a matrix patterns is widely used for video cameras and digital cameras. Such solid-state image sensing devices are roughly classified into CCD type and MOS type devices according to means for reading (taking out) photoelectric charges generated from a photoelectric converting element. A CCD type device stores photoelectric charges in a potential well and simultaneously transmits them, thus arising a disadvantage that a dynamic range is narrow. On the other hand, an MOS type device reads electric charges stored in a pn junction capacitance of a photodiode via an MOS transistor.
As a technique for widening a dynamic range of an MOS type solid-state image sensing device, the assignee of this patent application proposed a solid-state image sensing device which includes photocurrent generating means for generating a photocurrent according to an amount of incident light, an MOS transistor for inputting a photocurrent, and bias means for biasing the MOS transistor so that a sub-threshold current flows in the MOS transistor. In this device, a photocurrent is logarithmically converted (See Japanese Patent Laid-Open Publication No. H3-192764). Although such a solid-state image sensing device has a wide dynamic range, threshold properties of the MOS transistors provided for respective pixels are different from one another, thereby occasionally making sensitivity different per pixel. Therefore, it is necessary to hold an output obtained by previously emitting a bright light (uniform light) with uniform brightness as correcting data for correcting outputs of respective pixels at the time of taking an image of an object.
However, there arise problems such that it is complicated for an operator to irradiate the respective pixels using an external light source and exposure cannot be executed uniformly and effectively. Moreover, when a uniform light emitting mechanism is provided to an image sensing device, there arises a problem that the structure of the image sensing device becomes complicated. Therefore, as a technique to solve the problems, the assignee of this application proposed a solid-state image sensing device which is capable of counteracting sensitivity unevenness of each pixel without previously emitting an uniform light (See Japanese patent Laid-Open Publication No. 2001-094878).
According to a solid-state image sensing device with the structure described above, a switch is provided between a photodiode for performing a photoelectric converting operation and an MOS transistor configured to operate in a sub-threshold region. At the time of resetting, the switch electrically disconnects the photodiode and the MOS transistor, and a signal is output. The sensitivity unevenness of each pixel can be countered by using the signal, which is output at the time of resetting, as correcting data to counteract the sensitivity unevenness of each pixel. However, with such operation, when the switch connects them after a reset operation is finished, electric charges remain at a connecting part between the photodiode and the MOS transistor. As a result, since an image sensing operation is performed in the condition that the electric charges are remained, an after-image phenomenon occurs.
Further, to prevent the occurrence of the after-image phenomenon, the assignee of this application proposed a solid-state image sensing device configured to perform another reset operation on electric charges remained at the connecting node between a photodiode and an MOS transistor via the MOS transistor, after the MOS transistor is connected with the photodiode and a reset operation is finished (See Japanese patent Laid-Open Publication No. 2003-163841).
A structure of a pixel provided to the solid-state image sensing device is shown in FIG. 17. In accordance with the pixel in FIG. 17, a pn photodiode PD forms a photosensitive section (photoelectric converting section). A cathode of the photodiode PD is connected with a drain of an MOS transistor T1. A source of the MOS transistor T1 is connected with a drain and a gate of an MOS transistor T2 and a gate of an MOS transistor T3. A source of the MOS transistor T3 is connected with a drain of an MOS transistor T4 which is adapted to select lines. A source of the MOS transistor T4 is connected with an output signal line 16. Here, the respective MOS transistors T1 to T4 are P-channel MOS transistors.
A DC voltage VPD is applied to an anode of the photodiode PD and a drain of the MOS transistor T3. On the other hand, a signal φ VPS is input to a source of the MOS transistor T2. The signal φ VPS is switched among three voltage values VH, VM and VL (VL<VM<VH). Moreover, a signal φ S is input to a gate of the MOS transistor T1, and a signal φ V is input to a gate of the MOS transistor T4.
To the pixels with such structure, respective signals are provided according to a timing chart shown in FIG. 18A. That is, the signal φ S is brought into low level and the MOS transistor T1 is turned on while the voltage of the signal φ VPS is turn to VL. Then an image sensing operation is started. As the signal φ VPS is brought into VL in this way, the MOS transistor T2 begins to operate in a sub-threshold region only after the gate voltage reaches a predetermined value. Therefore, a linearly converted electric signal is output when a brightness value of an object is lower than the predetermined value and a logarithmically converted electric signal is output when the brightness value of the object is equal to or higher than the predetermined value. At Time ta1, a pulse signal φ V in low level is provided and the MOS transistor T4 is turned on. As a result, a signal during the image sensing operation is output to the output signal line as image data.
After the pulse signal φ V is brought into high level, at Time ta2, the signal φ S is brought into high level and the MOS transistor T1 is turned off to stop the image sensing operation. At Time ta3, the voltage of the signal φ VPS is brought into VH and a bias voltage to be given to the MOS transistor T2 is made to be higher than the bias voltage used during the image sensing operation to start resetting. As a result, as shown in FIG. 18B, the gate voltage Vg of the MOS transistor T2 reduces by Time ta2 according to the brightness of the object. That is, with higher brightness of the object, the gate voltage Vg becomes lower. As a reset operation starts at Time ta3, it begins to change so that the gate voltage Vg becomes high.
At Time ta4, a pulse signal φ V in low level is given, and a signal reflecting a threshold voltage of the MOS transistor T2 is output to the output signal line. This signal represents a sensitivity unevenness among the pixels and used as correcting data to correct the sensitivity unevenness. At Time ta5, the signal φ VPS is brought into VL and the signal φ V is brought into high level. At Time ta6, the signal φ S is brought into low level and the MOS transistor T1 is turned on. As a result, at Time ta6, the photodiode PD and the MOS transistor T2 are electrically connected to each other. With electric charges remained at the photodiode PD, the gate voltage Vg of the MOS transistor T2 reduces as shown in FIG. 18B, and an after-image phenomenon occurs.
At Time ta7, by setting the signal φ VPS as VM temporarily, the MOS transistor T2 is temporarily set to be conductive in the condition that the photodiode PD and the MOS transistor T2 are connected via the MOS transistor T1. With such structure, the electric charges, a cause of an after-image, remained at the connecting part between the cathode of the photodiode PD and the gate and drain of the MOS transistor T2 is discharged, and, as shown in FIG. 18B, the gate voltage Vg of the MOS transistor T2 becomes high. At Time ta8, the signal φ VPS is brought into VL and the image sensing operation is started.
As described above, a pixel having a structure illustrated in FIG. 17 is adapted to operate as shown in FIG. 18A so that the gate voltage Vg of the MOS transistor T2 changes as shown in FIG. 18B. Here, at the time of bringing the voltage of the signal φ VPS into VH to start a reset operation of the MOS transistor T2 at Time ta3, the gate voltage Vg of the MOS transistor T2 varies according to a brightness value of an object. Unevenness of the gate voltage Vg at the time of starting the reset operation (Time ta3) in accordance with the brightness value of the object causes unevenness of the voltage values of gate voltage Vg at respective Times ta6 to ta8. Therefore, the voltage just before starting the image sensing operation (Time ta8) varies in the respective pixels having different brightness values of the image sensed object.
FIG. 19 shows a typical example of a relation of a sensor surface illuminance (brightness value) X and output Y in a solid-state image sensing device including a pixel having a structure illustrated in FIG. 17. FIG. 19 magnifies and shows a low brightness region where the output Y of the solid-state image sensing device linearly changes according to an amount of incident light, and the sensor surface illuminance, the horizontal axis, is illustrated in liner scale. In FIG. 19, the output Y of the solid-state image sensing device represents the values after the sensitivity unevenness is corrected with the correcting data. Further, in FIG. 19, the continuous line represents a preferable photoelectric converting property (photoelectric converting property in which the relation of X and Y is Y=X1.0) based on values in which the output Y becomes a value proportional to the sensor surface illuminance (brightness value), and the dashed line represents a photoelectric converting property based on actual measured values of the output Y.
The actual photoelectric converting property illustrated with the dashed line in FIG. 19 is a property illustrated as a curved line representing a relation of Y=X0.65. Within a region where the solid-state image sensing device performs a linear converting operation, the actual measured value of the output Y becomes lower than the preferable value as the sensor surface illuminance (brightness value) becomes higher. As a result, an occurrence of an internal loss in the solid-state image sensing device is found. The reason for this seems that, in the low brightness region where linearly converted values are output, the gate voltage Vg of the MOS transistor T2 just before starting the image sensing operation (Time ta8) is higher than the gate voltage Vg of the MOS transistor T2 at the time of finishing a reset operation by turning off the MOS transistor T1 (Time ta6).
In other words, when the voltage of the signal φ VPS is brought into VM in order to prevent the after-image phenomenon, the voltage value VM is kept constant and not related to the brightness value, and the period of time to maintain the voltage value as VM is also kept constant. Therefore, when the signal φ VPS is brought into VM, a substantial change may occur depending on the brightness value of the image sensed object and the above internal loss may occur.